In many data transmission networks, there is a need to synchronize the clock signals of the network elements, so that the ratio of the clock signal frequencies of two or more separate network elements is constant, as accurately as possible. Often the momentary phases of clock signals of separate network elements are attempted to be kept mutually locked as accurately as possible. Said network elements can be for instance routers or base stations in a mobile phone network. FIG. 1 illustrates an example of a data transmission system with a need to mutually synchronize the clock signals of the base stations 101 and 102 of a mobile phone network. The base station 101 is connected to the network element 110 by a modem line 103, and the base station 102 is connected to the network element 110 by a modem line 104. Said modem lines can be for example ADSL (Asymmetric Digital Subscriber Line) or VDSL (Very high speed Digital Subscriber Line) lines. The modem line 103 includes modems 111 and 112 as well as a connecting data transmission cable 113 therebetween. Respectively the modem line 104 includes modems 114 and 115 as well as a connecting data transmission cable 116 therebetween. The modem line can also comprise two or several successive modem links, each of which is provided with a data transmission cable, a modem being arranged at both ends of the cable. The network element 110 is arranged to connect the modems 112 and 115 to a data network 120. The network element 110 and the modems 112 and 115 are often integrated as one DSLAM device (Digital Subscriber Line Access Multiplexer). The data network 120 can be for example the Internet.
In a prior art arrangement, the mutual synchronizing of the clock signals of the base stations 101 and 102 is based on synchronizing messages that are transmitted via modem lines 103 and 104 to the base stations 101 and 102, and on the basis of which each base station adjusts the operation frequency and/or operation phase of its own clockwork. A remarkable random-type share is often found in the transit delay of the synchronizing messages, and this random-type share complicates the adjusting process based on synchronizing messages and weakens the quality of the mutual synchronizing of the clock signals. Said random-type share results, among others, from random-type queuing delays experienced by the data to be transferred in the transmission and/or reception buffers of the network element 110 and of the modems 111, 112, 114 and 115. The random-type nature of the transit delay is strong particularly in packet, frame and cell switched data transmission.
The publications EP 1455473 A2 and WO 2005/020486 A1 suggest an arrangement where the clock signal are adjusted on the basis of the information represented only by those synchronizing messages that have experienced the shortest detected transit delay. Each synchronizing message that has experienced the shortest detected transit delay is selected during an observation period of a predetermined length from among the group of synchronizing messages. When the data network is lightly burdened, the synchronizing messages that have experienced the shortest detected transit delay have not needed to remarkably queue in the transmission buffers and/or reception buffers of network elements. In other words, said transmission buffers and/or reception buffers have been empty or nearly empty, when the synchronizing messages that have experienced the shortest detected transit delay have arrived in said buffers. Consequently, the arrangement can be used for reducing the effect of random-type queuing delays on the adjustment of the clock signal. The heavier the data network is burdened, the rarer are situations where the transmission buffers and/or reception buffers of the network element are empty or nearly empty. When the burdening of the data network increases, the shortest synchronizing message transit delay that occurs during an observation period with a fixed duration all the more rarely represents a transit delay that does not contain random-type queuing delay. The random-type share of the transit delay complicates the operation of adjustments based on synchronizing messages and deteriorates the quality of the mutual synchronizing of the clock signals. The random-type share of the shortest transit delay occurring during an observation period can be cut by extending the duration of the observation period. On the other hand, any lengthening of the observation period also extends the interval between successive adjusting operations, which complicates the operation of adjustments based on synchronizing messages and deteriorates the quality of the mutual synchronizing of the clock signals.
The publication WO 2005/077063 A2 suggests an arrangement where the adjusting effect of the information represented by a synchronizing message is attempted to be weighted according to how near the transit delay experienced by said synchronizing message is to the transit delay average, type value, median value or low-pass filtered value. Consequently, this solution reduces the effect of the extreme ends of the transit delay distribution on the clock signal adjustment. In certain situations, the method according to this arrangement results in an improved quality in the mutual synchronizing of clock signals, as compared to the conventional method based on averaging or low-pass filtering. The challenge in the practical implementation is the effect of the low-frequency components of the transit delays on the transit delay average, type value, median value and low-pass filtered value.
In another prior art arrangement, the mutual synchronizing of the clock signals of the base stations 101 and 102 is based on satellite transmitted messages that are received in the base stations 101 and 102, and on the basis of which each base station adjusts the operation frequency and/or operation phase of its own clockwork. Said satellite can belong for example to the US GPS system (Global Positioning System), to the European Galileo system or to the Russian GLONASS system. This kind of solution requires that there are provided receiver systems in connection with the base stations 101 and 102 for receiving synchronizing messages. Said receiver systems can take up a remarkable share of the price of the base stations. In addition, disturbance signals can for political reasons be added in the satellite transmitted messages, which often weakens the accuracy of the synchronizing quality.